Stress controlled electroless nickel deposits

ABSTRACT

An article of manufacture and a method of producing the article, the method including depositing a nickel-phosphorus alloy coating having a controlled residual stress therein on a metal substrate selected from the group consisting of beryllium, aluminum, titanium, iron, nickel, copper and their alloys. The method includes introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type and controlling the hypophosphite anion and nickel concentrations, the pH of the bath, and the temperature of the bath to control the phosphorus content of the nickel-phosphorus coating deposited on the metal substrate at a predetermined level depending on the metal substrate and thereafter removing the coated metal substrate from the bath and cooling the same to ambient temperatures to produce a metal substrate having deposited thereon a nickel-phosphorus coating having a controlled residual stress therein. Substantially residual-stressfree coatings may be produced and the coated article may be annealed and re-annealed.

United States Patent Parker et a1.

STRESS CONTROLLED ELECTROLESS NICKEL DEPOSITS June 3, 1975 Primary Examiner-Charles E. Van Horn Assistant Examiner-Caleb Weston Atzorney, Agent, or Firm-Prangley, Dithmar, Vogel, Sandler & Scotland [57] ABSTRACT An article of manufacture and a method of producing the article, the method including depositing a nickelphosphorus alloy coating having a controlled residual stress therein on a metal substrate selected from the group consisting of beryllium, aluminum, titanium, iron, nickel, copper and their alloys. The method includes introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type and controlling the hypophosphite anion and nickel concentrations, the pH of the bath, and the temperature of the bath to control the phosphorus content of the nickel-phosphorus coating deposited on the metal substrate at a predetermined level depending on the metal substrate and thereafter removing the coated metal substrate from the bath and cooling the same to ambient temperatures to produce a metal substrate having deposited thereon a nickel-phosphorus coating having a controlled residual stress therein. Substantially residual-stressfree coatings may be produced and the coated article may be annealed and re-annealed.

6 Claims, 5 Drawing Figures STRESS OF ELECTROLESS NICKEL ON STEEL, TITANIUM AND STAINLESS STEEL [75] Inventors: Konrad Parker, Park Ridge;

Hasmukhlal M. Shah, Chicago, both of Ill.

[73] Assignee: General American Transportation Corporation, Chicago, Ill.

[22] Filed: Oct. 1, 1970 [21] Appl. No.: 77,063

[52] US. Cl. t. 427/380; l48/6.l5 R [51] Int. Cl. C23c 1/10 [58] Field of Search 117/130 E, 160 R; 148/6.l5; 106/1 [56] References Cited UNITED STATES PATENTS 2,928,757 3/1960 Lee et a1. 117/130 E X 2,962,811 12/1960 Herbert 1 .4 117/130 E X 3,060,059 10/1962 Sickles t 117/130 E 3,070,982 1/1963 McGowan 1 r l 117/130 E X 3,088,846 5/1963 Lee 117/130 E X 3,113,035 12/1963 MacCormack 117/130 E X 3,129,502 4/1964 Olson 117/130 E X 3,152,009 10/1964 De bong 117/130 E D LU w 10 Z LLI l- 0 U) E 5, s

(7) -IO tn m a: o. 5 "-2 O O Q As p1oted --Anneoled 3O .0 STEEL IE1 TITANIUM :2 STAINLESS 7 PHOSPHORUS STRESS, ksi

STRESS ksi PATENTEUJUH 10 m5 TENSILE COMPRESSIVE COMPRESSIVE TENSILE sum 2 3887732 FIG.3

STRESS OF ELECTROLESS NICKEL 0N ALUMINUM, BRASS AND NICKEL 0O ALUMINUM I BRASS NICKEL As-pla'red Annealed 4 5 6 7 8 9 IO II I2 I3 PHOSPHORUS STRESS OF ELECTROLESS NICKEL ON STEEL, TITANIUM AND STAINLESS STEEL B--\ n IOO STEEL 1 In TITANIUM L STAINLESS 9 5 B--+ a sz h -Asp|a1ed --Anneoled 4 5 6 T 8 9 I0 II I2 I3 PHOSPHORUS PATENIED SHEET EFFECT OF DEPOSIT THICKNESS ON BOW-OUT OF ELECTROLESS NICKEL II2.3%P) PLATED BERYLLIUM STRIPS o A m A U 0 e H o 0 48 I w 4 O 5 w A m o m m 0 m m w 0 0 w o o o m w o m UZOU Xw ZOU DEPOSIT THICKNESS, in.

STRESS CONTROLLED ELECTROLESS NICKEL DEPOSITS This invention relates to a method for depositing on a metal substrate a nickel-phosphorus coating having a controlled residual stress therein, and the article pro duced by the method.

An important object of the present invention is to provide a method and article produced thereby, the method producing a nickel phosphorus alloy coating having a controlled stress therein and chemically deposited on a metal substrate selected from the group consisting of beryllium. aluminum, titanium, iron, nickel, copper, and their alloys, the method comprising introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type, maintaining the bath parameters at predetermined values to deposit on the metal substrate a uniform nickelphosphorus coating having a phosphorus content of a predetermined value in the range between about 13 percent and about 3 percent, the predetermined value of the phosphorus content of the alloy coating being inversely proportional to the thermal expansion coefficient for the metal substrate, whereby to produce a metal substrate having chemically deposited thereon a nickel-phosphorus coating having a controlled stress therein.

Another object of the present invention is to provide a method and article produced thereby, the method producing a nickel-phosphorus alloy coating having a controlled residual stress therein and chemically depos ited on a metal substrate selected from the group consisting of beryllium, aluminum, titanium. iron, nickel, copper and their alloys, the method comprising intro ducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type, the bath hav ing a hypophosphite ion concentration expressed in moles per liter of from about 0.1 to about 0.5, main taining the pH of the bath at a predetermined value in the range from about 3.5 to about 7 and the temperature of the bath at a predetermined value in the range from about 70C. to about 95C. to deposit on the metal substrate a nickel-phosphorus coating having a phosphorus content of a predetermined value in the range of between about 13 percent and about 3 percent, maintenance of the pH of the bath at the upper end of the pH range coupled with maintenance of the temperature of the bath at the lower end of the temperature range producing a coating having a phosphorus content at the lower end of the phosphorus range and maintenance of the pH of the bath at the lower end of the pH range coupled with maintenance of the temper ature of the bath near the upper end of the temperature range producing a coating having a phosphorus content at the upper end of the phosphorus range, a coating having a phosphorus content near the lower end of the phosphorus range tending to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compressive stress, contacting the metal substrate with the bath for a time period sufficient to deposit on the metal substrate a coating having a predetermined thickness of at least about one-tenth mil (0.0001 inch), removing the coated metal substrate from the bath and cooling the coated metal substrate to ambient temperatures, whereby to produce a metal substrate having chemically deposited thereon a nickel-phosphorus coating having a controlled residual stress therein.

Another object of the present invention is to provide a method and article produced thereby in which the coating and the metal substrate are annealed by heating the coated metal substrate to a temperature in the range from about 250F. to about 600F. and thereafter cooling to ambient temperatures.

Still another object of the present invention is to provide a method and article produced thereby in which the nickeLphosphorus coating is substantially residualstress free.

A further object of the present invention is to provide a method and article produced thereby in which the asplated nickel-phosphorus coating and the metal substrate are annealed to provide a coating that is substantially residual-stress free.

A still further object of the present invention is to provide a method and article produced thereby in which a beryllium substrate has deposited thereon an as-plated nickel-phosphorus alloy coating which is an nealed and reannealed, the re-annealed coating being substantially residual-stress free,

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings in which:

FIG. I is a graph illustrating the relationship between the phosphorus content of a nickel-phosphorus coating deposited on a metal substrate from a bath of the type employed in the present method and the bath tempera ture and bath pH;

FIG. 2 is a family of curves showing the relationship between the phosphorus content of a nickel phosphorus coating deposited on a beryllium substrate and the residual stress in the coating for as-plated coatings, annealed coatings and rc-annealed coatings;

FIG. 3 is a family of curves showing the relationship between the phosphorus content of a nickelphosphorus coating deposited on an aluminum, brass or nickel substrate and the residual stress in the coating for as-plated coatings and annealed coatings,

FIG. 4 is a family of curves showing the relationship between the phosphorus content of a nickelphosphorus coating deposited on a steel, titanium or stainless steel substrate and the residual stress in the coating for asplated coatings and annealed coatings; and

FIG. 5 is a family of curves showing the relationship between the thickness of the nickel-phosphorus coating and the bow-out of the coated metal strip for a beryllium substrate having a nickel-phosphorus coating deposited thereon containing 12.3 percent phosphorus by weight, the curves illustrating data for an as-plated coating and an annealed coating.

It is believed that internal stresses in metallic coatings result from atomic lattice defects and crystal distor tions formed during the deposition of the metallic coatings on the substrate. A metallic coating also can develop contractile or expansive stress if a differential volume change occurs between the coating and the substrate subsequent to the deposition of the coating on the substrate; therefore, the nature of the residual stress of the coating, that is the stress in the coating after the cooling thereof from the plating temperatures or annealing temperatures to ambient temperatures, is

most important with regard to the properties of the deposited coating. A high tensile residual stress can cause bending, cracking and blistering of the coating and also may increase porosity and stress corosion which in turn may decrease the fatigue life of high strength steel. Conversely. a slight compressive stress will improve the adhesion of the coating to the metal substrate. The residual stress of the coating is particularly important to deposits of relatively low ductility, such as elcctrolcss nickel. which deposits are especially susceptible to cracking when under high residual tensile stress.

It has been found that the residual stress of a nickelphosphorus coating deposited on a metal substrate may be controlled by varying the phosphorus content of the coating and for some substrates also by varying the coating thickness. The data for determining the resid ual stress of the various coatings were obtained by plating both sides of thin metal strips with a coating having a predetermined phosphorus content and plating to a predetermined thickness. Thereafter, the metal strips were inspected for flatness and one side was coated with a molten wax. After the wax had solidified, the strip was immersed in cold concentrated nitric acid which completely stripped the nickel-phosphorus coating from the unmasked, that is the unwaxed, side. The wax thereafter was removed with perchloroethylene before the bow out of the coated, thin metal strip was determined with an optical comparator. Any residual tensile stress in the surface of the nickel-phosphorus coating bends the metal strip to form a concave bow, while a residual compressive stress in the nickel phosphorus coating produces a convex bow in the thin metal strip. The degree of concave or convex bow-out is directly related to the magnitude of the residual ten sile (positive) or residual compressive (negative) stress. The bowout created by the residual stress in the nickel-phosphorus coating was measured at the center of the thin metal strips to an accuracy of one tenth of a mil with an optical comparator and thereafter the surface stress (S) of the nickel-phosphorus alloy was calculated for coatings deposited on beryllium, aluminum, titanium, steel. nickel, brass and stainless steel by use of the method developed by Soderburg and Graham as reported in Proc. Amer. Electroplaters Soc.. 34, 74 (l947). The Soderburg-Graham equation is based upon a strip rigidly held during plating and assumes that bending occurs only after deposition is completed. The overall stress S. in a compound beam, was calculated from the following equation:

S E l/r (c -d/2) dw in which 15,, Elastic modulus of substrate metal I Moment of inertia of effective section weighted to the base modulus, in psi I \\/3 [a (d+r)(al] 3 (H-ad) (di t-(Fl u Elastic modulus of coating,/Elastic modulus of basis metal c distance of neutral axis to outside surface fo deposit, inches tF+2dr+r /2 (ad+ r) d thickness of coating, inches r thickness of the strip, inches, w width of the strip. inches r radius of curvature, inches I 4L /8L I /8L (since L is much smaller than I) I iength of strip. inches L net change in bow-out, inches As the modulus of elasticity of electroless nickel as plated is 17 X 10 psi, and increases with heat treat ment to a maximum of 27 X l0 psi, at value of 20 X 10 was used in the stress calculations. The equation assumes that the bending of the plated strip is caused by two types of stress, one atributable to the structure of the deposited coating and the other attributable to the difference in thermal contraction of the substrate metal and the coating when the coated strip is removed from the hot plating bath and cooled to ambient temperatures. Another assumption of the equation is that the net force in the substrate acts in the center plane parallel to the neutral surface of the deposited coating. The stress in the coating at equilibrium (in the bent condition thereof) is calculated from the measured deforma tion of the strip. The original stress S, in the unflexed coating, is the sum of this calculated equilibrium force and the additional force necessary to return the coating alone to its unbent condition.

The electroless nickel plating bath used for most of the experiments contained herein was a proprietory bath of General American Transportation Corporation, identified by the registered trademark Kanigen, a typi cal bath including 0.09M NiSO,.6H-,O, 0.23M NaH PO .H O. and 0.30 M lactic acid in deionized water. Minor amounts of exaltants, stabilizers and wetting agents. all well known in the art, were also added. The pH of the bath was adjusted as needed with a 10% NaOH solution and measured to 0.05 units every half hour with a Beckman pH meter. a 10 liter Kanigen system being employed for plating the various metal strips. A complete system incorporates continuous circulation and filtration, as well as indirect heating of the plating solution by a steam heat exchanger.

The operating temperature of the bath was controlled to within i 1C. and the pH of the plating bath was controlled to within i 0.05. During the plating of the metal substrate, the concentration of all critical chemicals was maintained by frequent additions to the regeneration tank of the Kanigen system as required and indicated by hourly nickel and hypophosphite ion analysis. As is known in the art, the metal strips were cleaned with solvents and hot alkaline cleaner followed by a suitable pickle and etch prior to the plating thereof. Principally, the only variation in the bath composition was in the nickel compound, nickel sulphate, nickel chloride, and nickel sulfamate, all being used for the addition of nickel ions to the bath. The other compounds of the bath remained unchanged except for several deposits on nickel and steel substrates from an alkaline pyrophosphate bath. The only other significant change in the bath composition was to buffer the bath with sodium acetate or various ammonium salts in order to maintain the pH of the bath at a value in the range of from about 6 to about 7 to deposit nickelphosphorus coatings having a low phosphorus content in the range from about 4 percent to about 6 percent, all as hereinafter explained.

The operating parameters of the bath were maintained within the following ranges, the pH of the bath was maintained in the range of from about 3.5 to about 7. the preferred range being from about 4 to about 6, the temperature of the bath was maintained in the range of from about C. to about C., the preferred range being from about 82C. to about 95C., and the hypophosphite anion concentration expressed in moles per liter was maintained in the range of from about 0.! to about 0.5, the preferred concentration being from about 0.2 to about 0.4. In general, the bath temperature was maintained in the range of from about 93C. to about 95C. except when operating the bath at pHs toward the alkaline side, that is pH's in excess of about 5 at which times the temperature of the bath was maintained in the range of from about 82C. to about 84C. to provide better bath stability. The bath parameters are adjusted and maintained in order to deposit on the substrate a coating which due to its phsophorus content upon cooling will be substantially residual-stress free. When the rate at which the coating is deposited on the substrate decreases to less than about 0. 2 mils per hour, the rate may be increased by raising the bath tempera ture to a value of between about 93C. and 94C. which in turn may necessitate increasing the hypophosphite ion concentration of the bath expressed in moles per liter to about 0.4, the exact hypophosphite ion concen tration required depending to some extent on the desired phosphorus content of the coating.

The substrate metal strips were generally of two sizes, the beryllium substrate strips being about 3 inches long, about 0.25 inches wide and about 0.01 inches thick, the other substrate strips being about 6 inches long, about 0.5 inches wide and about 0.01 inches thick. All of the beryllium strips were milled and then stress relieved by chemical etching prior to introduction thereof into the plating bath. The length and width of each strip were determined to an accuracy of 0.01 inches and the thickness of each strip was determined to an accuracy of 0.000] inches. While most of the substrate strips were straight prior to plating, any initial deflection was measured with an optical comparator and recorded for later use in calculating the degree of bow-out.

Most of the beryllium strips were plated to a coating thickness of about four mils while most of the other substrate metals were plated to a coating thickness of about one mil, there being included herein other experiments showing the change in the residual stress of the coating with variation of the coating thickness, the coatings may be deposited to any desired thickness, the preferred thickness being in the range of from about 0.5 mils to about 10 mils. After the plated strips were removed from the bath, the phosphorus content of the coating was determined by spectrophotometric analysis of the nitric acid solutions used to remove the coating from one side of the plated substrates as hereinbefore described. After cooling to ambient temperatures, the residual stress of the coating was determined by measurement of the bow-out of the coated strip, all as hereinbefore explained. Some of the coated strips were thereafter annealed by heating the plated strips to temperatures in the range of from about 250F. to about 600F., the preferred method being to heat the plated strip to a temperature of about 375F. for a time period in the range of from about l hour to about 4 hours, oth ers of the coated substrates were annealed by heating to a temperature of about 375F. for about 1 hour and thereafter heating to a temperature of about 475F. for about 1 hour. After the annealing of the coated substrate, the strips were cooled to ambient temperatures and the residual stress of the coating was determined as hereinbefore explained. Some of the beryllium strips were re-annealed by re-heating the annealed, coated strip to a temperature of about 375F. for about 4 hours, the residual stress being calculated after the cooling of the strip to ambient temperatures, all as hereinbefore explained.

Referring to FIG. 1 of the drawings, it is seen that the phosphorus content of the electroless nickel coating can be varied from about 6 percent to about 12.5 per cent by varying the pH in the range of from about 4 to about 5 and varying the temperature in the range of from about 82C. to about 95C., buffering of the bath to a pH in the range of from about 6 to about 7 while maintaining the bath at a temperature of about 82C. resulting in deposition of a coating having a phosphorus content in the range of from about 4 percent to about 6 percent. The pH6 curve is of the same general character as the pHS curve but displaced to the left thereof as viewed in FIG. 1. Curve 10, the pHS curve in FIG. 1, shows that an increase in the bath temperature results in an increase in the phosphorus content of the deposited coating, whereas curves l5 and 20 representing baths having a constant pH of 4.5 and 4, respectively, show that decreasing the temperature of the bath increases the phosphorus content of the deposited coating. Curve 25 shows that a bath maintained at a constant temperature of 94C. can deposit coatings having increased phosphorus contents as the pH of the bath is decreased from about 5 to about 4. It is seen that by maintaining the pH of the bath at a predetermined value in the range of from about 3.5 to about 7 and the temperature of the bath at a predetermined value in the range of from about C. to about 95C., the phosphorous content of a nickel-phosphorus coating deposited on the substrate can be controlled in the range of from about 13 percent to about 3 percent, maintenance of the pH of the bath at the upper end of the pH range coupled with maintenance of the temperature of the bath at the lower end of the temperature range producing a coating having a phosphorus content at the lower end of the phosphorus range and maintenance of the pH of the bath at the lower end of the pH range coupled with maintenance of the temperature of the bath near the upper end of the temperature range producing a coating having a phosphorus content at the upper end of the phosphorus range.

EXPERIMENT l Beryllium strips of the dimensions hereinbefore set forth were introduced into an aqueous bath of the nickel cation-hypophosphite anion type, as hereinbefore described, for a time period sufficient chemically to deposit thereon a nickel-phosphorus alloy coating having a thickness of about 4 mils. The strips were re moved from the plating bath, dipped in molten wax as hereinbefore described, one side of the strip being stripped with nitric acid and thereafter the molten wax being removed with perchloroethylene, all as hereinbefore explained. Thereafter the strips were cooled to ambient temperatures and the residual stress determined by the methods set forth above. FIG. 2 shows the variation in the residual stress of the coating for coatings having different phosphorus contents. Curve 30 shows the changes in the residual stress of an as-plated coating, it being noted that an as-plated coating having a phosphorus content of about 8.5 percent is substantially residual-stress free. Curve 35 shows that an annealed coating having a phosphorus content slightly in excess of about l 1.5 percent on a beryllium substrate is substantially stress free, the annealing of the coating being accomplished by heating the as-plated beryllium substrate to a temperature of about 375F. for a time period of about 4 hours and thereafter cooling to ambient temperatures before the residual stress was ascer tained. Curve 40 shows that a re-annealed coating hav ing a phosphorus content of about 12 percent on a be ryllium substrate is substantially residual-stress free. the re-annealing of the coating being accomplished by heating the as-plated beryllium substrate to a temperature of about 375F. for a time period of about four hours to anneal the same, cooling the annealed beryllium substrate to ambient temperatures, reheating the cooled and annealed beryllium substrate to a temperature of about 375F. for a time period of about 4 hours to re-anneal the same and thereafter cooling to ambient temperatures.

Reference to FIG. 5 of the drawings shows the effect of the coating thickness on the bow-out of electroless nickel plated beryllium strips, the phosphorus content of the coating being maintained at a value of about 12.3 percent. Curve 105 shows that the bow-out varies from a value of about zero to a value of about 0.06 inches for a change in deposit thickness of from about 0.5 mils to about 4.3 mils, the curve 105 extending substantially as a straight line for coatings up to a thickness of about mils. The curve 110 shows the change in bow-out for a beryllium strip annealed by heating the strip to a temperature of about 375F. for a time period of about four hours, the curve 110 extending substantially as a straight line from the right-hand portion thereof as viewed in FIG. 5 for coatings up to a thickness of about 10 mils.

EXPERIMENT 2 Aluminum strips of the size previously indicated were plated with coatings having a phosphorus content in the range of from about 48 percent to about 12.4 percent and having a thickness in the range of from about 05 mils to about 4.0 mils. the substrates being aluminum 7075 except where indicated. The experimental data is set forth in Table 1 below.

Reference to Table l and FIG. 3 of the drawings shows that the residual stress of electroless nickel coatings deposited on aluminum substrates varies from about +16 to about 1 8 Ksi for as-plated coatings having phosphorus contents in the range of from about 4 percent to about 12.4 percent, but that the residual stress decreased with an increase in thickness of the coating. Reference to curve 45 shows that for a coating having a thickness of about one mil and a phosphorus content slightly in excess of about 5.5 percent an as plated nickeLphosphorus coating deposited on an aluminum substrate is substantially residual-stress free. Curve 50 shows that for a coating having a thickness of about one mil and a phosphorus content of about 5 percent an annealed nickel-phosphorus coating on an aluminum substrate is substantially residual-stress free, the annealing of the coating being accomplished by heating the as'plated aluminum substrate to a temperature of about 375F. for a time period of about one hour and thereafter cooling the annealed coated aluminum substrate to ambient temperatures. It is seen from the above. that annealing the nickel-phosphorus coating deposited on an aluminum substrate decreased the tensile stress and increased the compressive stress in the coatings. This result may be due to an expansion of the coating layer or to a slight contraction of the aluminum alloy. but this is offered by way of explanation only.

The coatings having low phosphorus contents in the TABLE 1 STRESS OF ELECTROLESS NICKEL ON ALUMINUM DEPOSIT BOW-OUT STRESS Thickinches Ksi ness lhos. As- Asmils 7r Plated Annealed Plated Annealed 0.5 8 3 -.l050 .2420 |4.1 -32.6 1.0 8.3 -.l450 -2255 -12.2 |9.0 1.5 8.3 .1832 ".3000 l2.5 20.4 3.0 8.3 -.2276 .3352 -l2.3 -l8.2 4.0 8.3 .2780 .310S -|4.6 16.3

0.6 12.4 1475 .l800 l 7.4 -2l.2 1.3 12.4 .2040 -.2693 14.9 -l9.6 3.0 12.4 .3325 .3962 17.5 2l.5

EXPERIMENT 3 Brass strips of the size previously indicated were plated with coating having a phosphorus content in the range of from about 6.7 percent to about 10.8 percent and having a thickness in the range of from about 1.0 mils to about 4.0 mils, the substrates being yellow brass. The experimental data is set forth in Table ll below.

Reference to Table II and to FIG. 3 of the drawings shows that the residual stress of electroless nickel coatings deposited on brass substrates varies from about 1 .1 Ksi tensile stress to about 3.9 Ksi compressive stress for phosphorus contents in the plated coatings of about 7 percent to about 11 percent. Reference to curve 55 shows that for a coating having a thickness of about 1 mil and a phosphorus content of about 7 percent an asplated nickel-phosphorus coating deposited on a brass substrate is substantially residual-stress free. Curve 60 shows that no annealed coating on a yellow brass substrate having a thickness of about 1 mil and a phosphorus content in the range shown is substantially residualstress free, the annealing of the coating being accomplished by heating the as-plated brass substrate to a temperature of about 375F. for a time period of about 1 hour and thereafter cooling the annealed coated brass substrate to ambient temperatures.

TABLE 11 STRESS OF ELECTROLESS NICKEL ON BRASS TABLE II -Continued STRESS OF ELECTROLESS NICKEL ON BRASS EXPERIMENT 4 Nickel strips of the size previously indicated were plated with coatings having a phosphorus content in the phosphorus coating on a nickel substrate is substantially residual-stress free. the annealing of the as-plated coating being accomplished by heating the as-plated coated nickel substrate to a temperature of about 375F. for a time period of about 1 hour and thereafter heating the coated nickel substrate to a temperature of about 475F. for a time period of about 1 hour and thereafter cooling the annealed coated nickel substrate to ambient temperatures. It is seen from the above that annealing nickel-phosphorus coatings deposited on nickel substrates increases the tensile stress of coatings having phosphorus contents of about 4 percent but decreases the tensile stress of coatings having phosphorus contents of about 7 percent to about 1 l percent. The stress in the 12.2 percent phosphorus coating is reversed from compressive to tensile upon annealing which indicates that the respective electroless nickel coatings behave differently during annealing. the very low and the very high phosphorus-containing coatings seem to shrink while in the middle range. that is from about 7 to l l percent phosphorus content. the coatings expand to attain a low compressive stress condition.

TABLE lll STRESS OF ELECTROLESS NICKEL ON NICKEL BATH DEPOSIT BOW-OUT STRESS Thickinches Ksi ness Phos. As- Asmils 9t Plated Annealed Plated Annealed Pyrophosphate 0.5 4.0 +2840 +2940 +687 +7l .l Pyrophosphate l .0 4.0 +3150 +5400 +4 I .7 +7 I .4 Pyrophosphatc 2.0 4.0 +3925 +6145 +3 I .2 +488 Pyrophosphate 3.0 +2565 +3960 +163 +252 Sulfate l0 6.7 +0455 +0030 6.0 0.4 Sulfate 4.0 6.9 411740 .0740 +l00 4.2 Sulfarnate 0.5 9.5 +0110 0|70 2.7 4.] Sulfamate l.0 9.5 +0250 -.0 l 3.3 20 Sulfamate 2.0 9.5 +0595 .(J0o0 4.7 0.5 Sulfamate 3.0 9.5 +0870 +0535 5.5 3.4 Chloride 0.5 9.8 +0340 +0220 8.2 5.3 Chloride l.() 9.8 +0560 .0220 7.4 2.9 Chloride 3.0 9.8 +.l I .005(] 7.4 A 0.3 Sulfate 0.5 I03 +0285 .0020 6.) w 0.5 Sulfate l.0 10.3 +0215 .0050 2.8 0.7 Sulfate 0.5 1 L5 -.0020 .0030 v 0.5 0.7 Sulfate l0 ll.5 .0080 .0|25 l.l 1.7 Sulfate 1.5 l L5 .(J200 +0090 l9 0.9 Sulfate 2.0 ll.5 .03b0 +0620 2.9 4.) Sulfate 3.0 l l.5 .0480 +0500 3.1 3.2 Sulfate l .0 12.2 -.()430 +02 l 0 5.7 2.8 Sulfate 2.0 12.2 .0940 f 7.5

Sulfate 3.0 12.2 I230 +1870 7.8 +1 I.)

range of from about 4 percent to about 12.2 percent EXPERIMENT 5 and having a thickness in the range of from about 0.5 mils to about 4.0 mils, the substrates being nickel 200. The experimental data is set forth in Table III below.

Reference to Table Ill and FIG. 3 of the drawings shows that the residual stress of electroless nickel coatings deposited on nickel substrates vary from about 68.7 Ksi tensile stress to about 7.8 Ksi compressive stress for coatings having phosphorus contents from about 4 percent to about 12.2 percent but with the exception of the coatings deposited from pyrophosphate and chloride baths all the coatings show an increased residual stress with an increase in the coating thickness. Reference to curve shows that for a coating having a thickness of about l mil and a phosphorus content of about 10.8 percent, an as-plated nickel-phosphorus coating deposited on a nickel substrate is substantially residual-stress free. Curve shows that for a coating having a thickness of about 1 mil and a phosphorus content of about 9 percent. an annealed nickel- Stainless steel strips of the size previously indicated were plated with coatings having a phosphorus content in the range of from about 6.7 percent to about 1 1.5 percent and having a thickness in the range of from about 0.5 mils to about 3.0 mils. the substrates being 304. l88 stainless steel. The experimental data is set forth in Table IV below.

Reference to Table IV and FIG. 4 of the drawings shows that the residual stress of electroless nickel coat ings deposited on stainless steel substrates varies from about 2.6 Ksi tensile to about 2.7 Ksi compressive stress for a phosphorus content in the range of between about 6.7 percent and l 1.5 percent. but that the residual tensile stress in the asplated coatings decreases with an increase in coating thickness. Reference to curve shows that for a coating having a thickness of about one mil and a phosphorus content of about 9.6 percent an as-plated nickelphosphorus coating deposited on a stainless steel substrate is substantially residual-stress free. Curve 80 shows that for annealed coatings having a thickness of about l mil and a phosphorus content in the range disclosed. no coating is substantially residualstres free, the annealing being accomplished by heating the as-plated stainless steel substrate to a temperature of about 375F. for a time period of about 1 hour and thereafter cooling the coated stainless steel substrate to ambient temperatures. It is seen from the above that annealing the nickel-phosphorus coating deposited on a stainless steel substrate decreases the tensile stress and increases the compressive stress in the coatings.

TABLE [V STRESS OF ELECTROLESS NICKEL ON 18-8 STAINLESS STEEL DEPOSIT BOWOUT STRESS Thickinches Ksi ness Phos. As Asmils I? Plated Annealed Plated Annealed 1.0 6.7 +0210 .0070 +2.6 0.9 1.0 7.2 +0240 .0655 +3.0 8.2 1.0 8.7 -.0045 +0250 0.6 3.1 0.5 10.3 +0090 ".01 Ill +2.1 2.5 1.0 10.3 .0063 .0l77 -0.t 2.2 1.0 11.5 .0105 .0l95 1.3 2.4 3.0 1 L5 .0445 +0027 2.7 -02 EXPERIMENT 6 Ksi tensile stress to about 9 Ksi compressive stress for phosphorus contents from about 4 percent to about 12.4 percent for as-plated coatings, but that the tensile stress of the coatings increases with an increase in thickness of those coatings with low phosphorus content and the compressive stress increases with an increase in thickness of those coatings with high phosphorus content. Reference to curve 85 shows that for a coating having a thickness of about one mil and a phosphorus content slightly in excess of 10 percent an as-plated nickel-phosphorus coating deposited on a steel substrate is substantially residual-stress free. Curve 90 shows that for a coating having a thickness of about one mi] and a phosphorus content of about 1 1.3 percent, an annealed nickel-phosphorus coating on a steel substrate is substantially residual-stress free. the annealing of the coating being accomplished by heating the as-plated steel substrate to a temperature of about 375F. for a time period of about 1 hour and thereafter heating the coated steel substrate to a temperature of about 475F. for a time period of about 1 hour and thereafter cooling the annealed coated steel substrate to ambient temperatures. It is seem from the above that annealing the nickel-phosphorus coating deposited on a steel substrate produces a slight decrease in the tensile stress of the coatings having a phosphorus content in the range offrom about 6.9 percent to about 9.8 percent and a slight decrease in the compressive stress for the coatings having a phosphorus content in the range of from about 11.5 percent to about 12.4 percent and a larger increase in the tensile stress of coatings having a phosphorus content of either 4 percent of about 10.3 percent. It is noted, however, that the coating having a phosphorus content of about 10.3 percent was deposited from a bath made up with ammonium hydroxide instead of sodium hydroxide, which ammonium bath uniformly deposited coatings having significant higher tensile stresses therein.

TABLE V STRESS OF ELECTROLESS NICKEL ON STEEL BATH DEPOSIT BOWOUT STRESS Thickinches Ksi ness Phos. As- Asmils 71 Plated Annealed Plated Annealed Pyrophosphate 1.0 4.0 +1930 +2150 +255 +284 Pyrophosphate 2.0 4.0 +2290 +2320 +182 +184 Pyrophosphate 3.0 4.0 +4485 +5955 +286 +379 Sulfate 0.5 6.9 +0150 +0105 3.6 2.5 Sulfate 1.0 6.9 +0350 +0290 4.6 3.8 Sulfate 4.0 6.9 +1620 +0350 9.3 2.0 Sulfate 1.0 8.1 +0270 +0320 3.5 4.2 Sulfate 2.0 8.1 +0500 +0330 3.9 2.5 Sulfate 3.0 8.1 +0770 +0440 4.9 2.8

Sulfamate 0.5 9.5 +0188 +0070 4.4 1.7 Sulfamate 1.0 9.5 +0340 +0100 4.5 1.3 Sulfamate 2.0 9.5 +0520 +0165 4.3 0.9 Sulfamate 3.0 9.5 +0816 +0455 5.2 2.9

Chloride 0.5 9.8 +0175 +0090 4.2 2.2 Chloride 1.0 9.8 +0420 +0125 5.6 1.7 Chloride 2.0 9.8 +0870 +0270 6.9 2.1

Ammonium 1.0 10.3 +0275 +0580 3.6 7.7

Sulfate 1.0 11.5 +0120 -.0070 1.3 0.8 Sulfate 3.0 1 1.5 .0245 +0080 1.3 +0.4 Sulfate 4.0 1 1.5 -.0320 +0080 1.6 +0.4 Sulfate .0 1 1.5 +0500 +0180 2.4 0.8 Sulfate 0.75 12.4 .0620 +0210 3.4 1.2 Sulfate 1.4 12.4 .0860 +0125 8.7 1.3 Sulfate 2.4 12.4 1020 +0725 7.3 5.2

EXPERIMENT 7 Titanium strips of the size previously indicated were plated with coatings having a phosphorus content in the range of from about 6.7 percent to about 12.4 percent and having a thickness in the range of from about 0.5 mils to about 4.1 mils, the substrates being Ti-6 Al4V. The experimental data is set forth in Table V] below.

Reference to Table VI and FIG. 4 of the drawings shows that the residual stress of electroless nickel coatings deposited on titanium substrates varies from about 8 Ksi tensile stress to about 3 Ksi compressive stress for as-plated coatings having phosphorus contents from about 6.7 percent to about 12.4 percent, but that the residual tensile stress decreases with an increase in the thickness of the coating. Reference to curve 95 shows that for a coating having a thickness of about 1 mil and a phosphorus content in the range of from about 6.7 percent to about 12.4 percent, no coating is substantially residual stress free, but reference to Table VI shows that a coating having a phosphorus content of about l2.4 percent and a thickness of about 1% mils on a titanium substrate is substantially residual-stress free. Curve 100 shows that no annealed coating having thickness of about 1 mil and a phosphorus content in the range of from about 6.7 percent to about 12.4 percent on a titanium substrate is substantially residualstress free, the annealing of the coating being accomplished by heating the as-plated titanium substrate to a temperature of about 375F. for a time period of about 1 hour and thereafter heating the coated titanium substrate to a temperature of about 475F. for a time period of about 1 hour and thereafter cooling the annealed coated titanium substrate to ambient temperatures. It is seen from the above that annealing the nickel-phosphorus coating deposited on a titanium substrate substantially increases the tensile stress of the coating.

TABLE VI STRESS OF ELECTROLESS NICKEL ON TITANIUM TABLE VI Continued STRESS OF ELECTROLESS NICKEL ON TITANIUM The internal stresses of electroless nickel deposits de pend upon the particular metal substrate on which the electroless nickel is plated. For metal substrates having a high thermal expansion coefficient, such as aluminum, the internal stresses of the electroless nickel tend to be compressive and for metal substrates having a relatively low thermal expansion coefficient, such as titanium, the internal stresses of the electroless nickel tend to be tensile. With a reported thermal expansion coefficient of 13 X l0'in/in/C the nickel alloy layer contracts more than substrates such as titanium or beryllium, but considerably less than aluminum or copper alloys such as brass or iron alloys such as stainless steel. Thus, while cooling from a plating temperature of about 95C. to ambient temperatures, a titanium substrate will shrink about 0.06 percent and an aluminum substrate will shrink about 0. l 3 percent as compared to a contraction or shrinkage of about O.l percent for the electroless nickel alloy coating. Due to this differential shrinkage. a tensile (contractile) stress develops on titanium substrates, while a compressive (expansive) stress develops on copper alloys such as brass and aluminum after the plating substrates have been removed DEPOSIT BOW-OUT STRESS from the hot plating bath and cooled to ambient tem- Thickinches Ksi peratul-es ness Phos. As- Asmils Hated Annealed plated Annealed Reference to Table Vll shows the difference In thermal expansion coefficient for various substrates and 67 +0500 +73 also shows that the phosphorus content of the alloy 05 +0330 +0660 H69 coating is inversely proportional to the thermal expanl.0 8.l H0360 +1200 +5.2 +l 7.4 ion oefficignt of e m 5 8- +0330 +2600 +2.4 +186 I th etal substrate for coatings hav M 8'] +0390 +2200 +27 +15 mg low residual stresses therein.

TABLE Vll STRESS OF ELECTROLESS NICKEL DEPOSITS 0n01 in. thick) SUBSTRATE MODULUS THERMAL DEPOSIT OF EXPANSION ELASTlClTY COEFFICIENT STRESS, Ksi

l0 psi inJirt/"C X l0 7rP As-Plated Annealed 4.0 +l5.7 +8.6 Aluminum 10 23 6.2 0.5 -4.9 Al 7075 T-6 8.3 l2.2 l9.0 l2.4 l7.0 2l.0 Brass I5 20 6.7 H13 45 7.0 (].2 -l0.5 7.2 (J.2 -8.7 8.4 -l.4 7.6 l(l.8 3.9 6.0 lit-B Stainless 28 17 6.7 +2.6

TABLE Vll Continued STRESS OF ELECTROLESS NICKEL DEPOSITS SUBSTRATE MODL'LUS THERMAL DEPOSIT OF EXPANSION ELASTlClTY COEFFlClENT STRESS, Ksi

X I(l' psi in./in./C X it) )lP AsPlutcd Annealed Nickel 30 l3 6.7 Hill +0.4 i +2.8 U.7 l 1.5 l. l l .7 I21 5.7 +2.8 Steel 30 ll 6.9 +4.6 +3.8 llO'JU) b l +3.5 +4.: I L 1.3 -(l,i 12.4 -41) *Ll Beryllium 44 |l.7 7.8 +lll.Z +61 8.6 +z 9 +4.9 10.3 .7 +0.6 11.3 -).5 +2.]

Titanium 16.5 o 7 +7.3 +l7.l Ti (1 Al 4V bl +5.1 +l7.4 l 1.5 +1.7 +7.3 11-1 +1.6 +6.6

In general, it is seen, therefore. that the residual stress of a nickel-phosphorus coating deposited on a metal alloy is controlled by varying the thickness in the range of from about 0.1 mils to about l0 mils and varying the phosphorus content of the coating in the range of from about 3 percent to about 13 percent. a coating having a phosphorus content near the lower end of the phosphorus range tending to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compressive stress. Annealing of the electroless nickelphosphorus coatings generally will reduce tensile stresses on substrates of aluminum. brass. steel and nickel but will increase the tensile stress of electroless nickel coatings deposited on substrates of titanium and beryllium.

In view of the foregoing, it is apparent that there has been provided a method and article produced thereby for depositing on a metal substrate a nickel-phosphorus coating having a controlled stress therein. While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A method of producing a nickel-phosphorus alloy coating having a predetermined and controlled residual stress in the range of from about +15 ksi tensile stress to about 17 ksi compressive stress therein and chemically deposited on a metal substrate selected from the group consisting of beryllium, aluminum, titanium. iron. nickel. copper and their alloys. said method com prising introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type, the bath having a hypophosphite ion concentration expressed in moles per liter of from about 0.l to about 0.5, maintaining the pH of the bath at a predetermined value in the range from about 3.5 to about 7 and the temperature of the bath at a predetermined value in the range from about 70C. to about 95C. to deposit on the metal substrate a nickeLphosphorus coating having a phosphorus content of a predetermined value in the range of between about 13 percent and about 3 percent, maintenance of the pH of the bath at the upper end of the pH range coupled with maintenance of the temperature of the bath at the lower end of the temperature range producing a coating having a phosphorus content at the lower end of the phosphorus range and maintenance of the pH of the bath at the lower end of the pH range coupled with maintenance of the temperature of the bath near the upper end of the temperature range producing a coating having a phosphorus content at the upper end of the phosphorus range. a coating having a phosphorus content near the lower end of the phosphorus range tending to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compres sive stress, contacting the metal substrate with the bath for a time period sufficient to deposit on the metal substrate a coating having a predetermined thickness of at least about one-tenth mil, removing the coated metal substrate from the bath and cooling the coated metal substrate to ambient temperatures, whereby to produce a metal substrate having chemically deposited thereon a nickel-phosphorus coating having a predetermined and controlled residual stress therein.

2. A method of producing an annealed coating having a predetermined and controlled residual stress in the range of from about +19 ksi tensile stress to about 2l ksi compressive stress therein deposited on a metal substrate selected from the group consisting of beryllium, aluminum, titanium. iron, nickel, copper and their alloys, said method comprising introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type. the bath having a hypophosphite ion concentration expressed in moles per liter of from about 0.1 to about 0.5, maintaining the pH of the bath at a predetermined value in the range from about 3.5 to about 7 and the temperature of the bath at a predetermined value in the range from about C. to about C. to deposit on the metal substrate a nickel-phosphorus coating having a phosphorus content of a predetermined value in the range of between about 13 percent and about 3 percent, maintenance of the pH of the bath at the upper end of the pH range coupled with maintenance of the temperature of the bath at the lower end of the temperature range producing a coating having a phosphorus content at the lower end of the phosphorus range and maintenance of the pH of the bath at the lower end of the pH range coupled with maintenance of the temperature of the bath near the upper end of the temperature range producing a coating having a phosphorus content at the upper end of the phosphorus range, a coating having a phosphorus content near the lower end of the phosphorus range tend ing to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compressive stress, contacting the metal substrate with the bath for a time period sufficient to deposit on the meta] substrate a coating having a predetermined thickness of at least about one-tenth mil, whereby to produce a metal substrate having chemically deposited thereon a nickel-phosphorus coating having a controlled stress therein, heating the coated metal substrate to a temperature in the range from about 250F. to about 600F. for a time period of at least about one hour, and cooling the coated metal substrate to ambient temperatures, where by to produce a metal substrate having thereon a coating having a predetermined and controlled residual stress therein.

3. The method set forth in claim 2, wherein the coated metal substrate is heated to a temperature of about 375F. for a time period up to about 4 hours.

4. The method set forth in claim 2, wherein the coated metal substrate is heated to a temperature of about 475F. for a time period up to about 4 hours.

LII

5. The method set forth in claim 2, wherein the coated metal substrate is heated to a temperature of about 375F. for at least about 1 hour and thereafter heated to a temperature of about 475F. for at least about I hour.

6. A method of producing a re-annealed substantially residual-stress-free coating on a beryllium substrate, said method comprising introducing the beryllium substrate into an aqueous bath of the nickel cationhypophosphite ion concentration expressed in moles per liter of about 0.4, maintaining the pH of the bath at about 4 and the temperature of the bath at about 82C. to deposit on the beryllium substrate a nickelphosphorus near the upper end of the temperature range producing a coating having a phosphorus content at the upper end of the phosphorus range, a coating having a phosphorus content near the lower end of the phosphorus range tending to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compressive stress, contacting the metal substrate with the bath for a time period sufficient to deposit on the metal substrate a coating having a predetermined thickness of at least about one-tenth mil, whereby to produce a metal substrate having chemically deposited thereon a nickelphosphorus coating having a controlled stress therein, heating the coated metal substrate to a temperature in the range from about 250F. to about 600F. for a time period of at least about I hour, and cooling the coated metal substrate to ambient temperatures, whereby to produce a metal substrate having thereon a coating having a predetermined and controlled residual stress 

1. A METHOD OF PRODUCING A NICKEL-PHOSPHORUS ALLOY COATING HAVING A PREDETERMINED AND CONTROLLED RESIDUAL STRESS IN THE RANGE OF FROM ABOUT +15 KSI TENSILE STRESS TO ABOUT -17 KSI COMPRESSIVE STRESS THEREIN AND CHEMICALLY DEPOSITED ON A METAL SUBSTRATE SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, ALUMINUM, TITANIUM, IRON, NICKEL, COPPER AND THEIR ALLOYS, SAID METHOD COMPRISING INTRODUCING THE METAL SUBSTRATE INTO AN AQUEOUS BATH OF THE NICKEL CATION-HYPOPHOSPHITE ANION TYPE, THE BATH HAVING A HYPOPHOSPHITE ION CONCENTRATION EXPRESSED IN MOLES PER LITER OF FROM ABOUT 0.1 TO ABOUT 0.5, MAINTAINING THE PH OF THE BATH AT A PREDETERMINED VALUE IN THE RANGE FROM ABOUT 3.5 TO ABOUT 7 AND THE TEMPERATURE OF THE BATH AT A PREDETERMINED VALUE IN THE RANGE FROM ABOUT 70*C. TO ABOUT 95*C. TO DEPOSIT ON THE METAL SUBSTRATE A NICKELPHOSPHORUS COATING HAVING A PHOSPHORUS CONTENT OF A PREDETERMINED VALUE IN THE RANGE BETWEEN ABOUT 13 PERCENT AND ABOUT 3 PERCENT, MAINTENANCE OF THE PH OF THE BATH OF THE UPPER END OF THE PH RANGE COUPLED WITH MAINTENANCE OF THE TEMPERATURE OF THE BATH AT THE LOWER END OF THE TEMPERATURE RANGE PRODUCING A COATING HAVING A PHOSPHORUS CONTENT AT THE LOWER END OF THE PHOSPHORUS RANGE AND MAINTENANCE OF THE PH OF THE BATH AT THE LOWER END OF THE PH RANGE COUPLED WITH MAINTENANCE OF THE TEMPERATURE OF THE BATH NEAR THE UPPER END OF THE TEMPERATURE RANGE PRODUCING A COATING HAVING A PHOSPHORUS CONTENT AT THE UPPER END OF THE PHOSPHORUS RANGE, A COATING HAVING A PHOSPHORUS CONTENT NEAR THE LOWER END OF
 1. A method of producing a nickel-phosphorus alloy coating having a predetermined and controlled residual stress in the range of from about +15 ksi tensile stress to about -17 ksi compressive stress therein and chemically deposited on a metal substrate selected from the group consisting of beryllium, aluminum, titanium, iron, nickel, copper and their alloys, said method comprising introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type, the bath having a hypophosphite ion concentration expressed in moles per liter of from about 0.1 to about 0.5, maintaining the pH of the bath at a predetermined value in the range from about 3.5 to about 7 and the temperature of the bath at a predetermined value in the range from about 70*C. to about 95*C. to deposit on the metal substrate a nickel-phosphorus coating having a phosphorus content of a predetermined value in the range of between about 13 percent and about 3 percent, maintenance of the pH of the bath at the upper end of the pH range coupled with maintenance of the temperature of the bath at the lower end of the temperature range producing a coating having a phosphorus content at the lower end of the phosphorus range and maintenance of the pH of the bath at the lower end of the pH range coupled with maintenance of the temperature of the bath near the upper end of the temperature range prOducing a coating having a phosphorus content at the upper end of the phosphorus range, a coating having a phosphorus content near the lower end of the phosphorus range tending to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compressive stress, contacting the metal substrate with the bath for a time period sufficient to deposit on the metal substrate a coating having a predetermined thickness of at least about one-tenth mil, removing the coated metal substrate from the bath and cooling the coated metal substrate to ambient temperatures, whereby to produce a metal substrate having chemically deposited thereon a nickel-phosphorus coating having a predetermined and controlled residual stress therein.
 2. A method of producing an annealed coating having a predetermined and controlled residual stress in the range of from about +19 ksi tensile stress to about -21 ksi compressive stress therein deposited on a metal substrate selected from the group consisting of beryllium, aluminum, titanium, iron, nickel, copper and their alloys, said method comprising introducing the metal substrate into an aqueous bath of the nickel cation-hypophosphite anion type, the bath having a hypophosphite ion concentration expressed in moles per liter of from about 0.1 to about 0.5, maintaining the pH of the bath at a predetermined value in the range from about 3.5 to about 7 and the temperature of the bath at a predetermined value in the range from about 70*C. to about 95*C. to deposit on the metal substrate a nickel-phosphorus coating having a phosphorus content of a predetermined value in the range of between about 13 percent and about 3 percent, maintenance of the pH of the bath at the upper end of the pH range coupled with maintenance of the temperature of the bath at the lower end of the temperature range producing a coating having a phosphorus content at the lower end of the phosphorus range and maintenance of the pH of the bath at the lower end of the pH range coupled with maintenance of the temperature of the bath near the upper end of the temperature range producing a coating having a phosphorus content at the upper end of the phosphorus range, a coating having a phosphorus content near the lower end of the phosphorus range tending to be in tensile stress and a coating having a phosphorus content near the upper end of the phosphorus range tending to be in compressive stress, contacting the metal substrate with the bath for a time period sufficient to deposit on the metal substrate a coating having a predetermined thickness of at least about one-tenth mil, whereby to produce a metal substrate having chemically deposited thereon a nickel-phosphorus coating having a controlled stress therein, heating the coated metal substrate to a temperature in the range from about 250*F. to about 600*F. for a time period of at least about one hour, and cooling the coated metal substrate to ambient temperatures, whereby to produce a metal substrate having thereon a coating having a predetermined and controlled residual stress therein.
 3. The method set forth in claim 2, wherein the coated metal substrate is heated to a temperature of about 375*F. for a time period up to about 4 hours.
 4. The method set forth in claim 2, wherein the coated metal substrate is heated to a temperature of about 475*F. for a time period up to about 4 hours.
 5. The method set forth in claim 2, wherein the coated metal substrate is heated to a temperature of about 375*F. for at least about 1 hour and thereafter heated to a temperature of about 475*F. for at least about 1 hour. 